1. Field of the Invention
The present invention relates to active solid polymer electrolyte membranes in solid polymer type fuel cells and a process for the production thereof.
2. Description of the Prior Art
With regard to active solid polymer electrolyte membranes of this type in the art, those in which a precious metal catalyst is supported on the surface of a solid polymer electrolyte membrane by a sputtering method are known.
However, since the conventional precious metal catalyst is formed as a layer, the conduction of the generated hydrogen ions to the solid polymer electrolyte membrane and the conduction from the electrolyte membrane to the air electrode are comparatively low, and the area of the interface at which the three elements of precious metal catalyst, solid polymer electrolyte membrane and fuel gas (hydrogen and air) come into contact with each other, that is to say, the area of the three phase interface, is small and in spite of a large amount of precious metal being supported on the electrolyte membrane the ability of the fuel cell to generate power is low, which is a problem.
It is an object of the present invention to provide an active solid polymer electrolyte membrane that can enhance the ability of the fuel cell to generate power with only a small amount of precious metal supported.
In accordance with the present invention in order to achieve the aforementioned object, an active solid polymer electrolyte membrane in a solid polymer type fuel cell comprises a solid polymer electrolyte membrane and a plurality of precious metal catalyst particles supported on the surfaces of the above-mentioned solid polymer electrolyte membrane by ion exchange and distributed uniformly over the surfaces thereof.
In accordance with the aforementioned arrangement, since the precious metal catalyst particles are present as spots on the surfaces of the aforementioned electrolyte membrane, the conduction of the generated hydrogen ions to the aforementioned electrolyte membrane and the conduction thereof from the electrolyte membrane to the air electrode both become high and association between the hydrogen ions and oxygen is enhanced. Moreover, the area of the three phase interface at which the three elements of precious metal catalyst particle, solid polymer electrolyte membrane and fuel gas come into contact with each other increases. Thus, it becomes possible to reduce the amount of precious metal supported on the aforementioned electrolyte membrane and at the same time increase the efficiency of power generation in the fuel cell.
Furthermore, in accordance with the present invention, an active solid polymer electrolyte membrane in a solid polymer type fuel cell is provided which comprises a solid polymer electrolyte membrane and a plurality of precious metal catalyst particles supported inside the surface layers of the above-mentioned solid polymer electrolyte membrane by ion exchange and distributed uniformly throughout the inside of the surface layers thereof.
In accordance with the aforementioned arrangement, since the precious metal catalyst particles are present as spots inside the surface layers of the aforementioned electrolyte membrane, the conduction of the generated hydrogen ions to the aforementioned electrolyte membrane and the conduction thereof from the electrolyte membrane to the air electrode become both high and association between the hydrogen ions and oxygen is enhanced. Moreover, the area of the three phase interface at which the three elements of precious metal catalyst particle, solid polymer electrolyte membrane and fuel gas come into contact with each other increases. It thus becomes possible to reduce the amount of precious metal supported in the aforementioned electrolyte membrane and at the same time increase the efficiency of power generation in the fuel cell.
Furthermore, it is an object of the present invention to provide a production process in which the aforementioned active solid polymer electrolyte membrane can be mass produced.
In accordance with the present invention in order to achieve the aforementioned object, there is proposed a process for the production of an active solid polymer electrolyte membrane in a solid polymer type fuel cell comprising a solid polymer electrolyte membrane and a plurality of precious metal catalyst particles supported on the surfaces of the above-mentioned solid polymer electrolyte membrane by ion exchange and distributed uniformly over the surfaces thereof, wherein the process comprises in sequence a step in which the aforementioned solid polymer electrolyte membrane is immersed in a solution of a precious metal complex so as to carry out ion exchange, a step in which the aforementioned solid polymer electrolyte membrane is washed with pure water, a step in which the aforementioned solid polymer electrolyte membrane is subjected to a reduction treatment, a step in which the aforementioned solid polymer electrolyte membrane is washed with pure water and a step in which the aforementioned solid polymer electrolyte membrane is dried.
The solid polymer electrolyte membranes which are known at present are polymer ion exchange membranes. Therefore, when the aforementioned ion exchange is carried out, precious metal complex ions are adsorbed on a plurality of ion exchange sites which are present on the surfaces of the aforementioned solid electrolyte membrane and are distributed uniformly over the surfaces thereof. In the first washing step, free precious metal complex ions which are present inside the aforementioned electrolyte membrane are removed and recovered. In the reduction step, groups of atoms bonded to the precious metal atoms of the precious metal complex ions are removed. In the second washing step the reducing component is removed from the aforementioned electrolyte membrane, and an active solid polymer electrolyte membrane can be obtained via the subsequent drying step.
When the reduction treatment is effected without carrying out the first washing, free precious metal atoms remain inside the aforementioned electrolyte membrane, and as these precious metal atoms generally do not contribute to the generation of hydrogen ions expensive precious metal is therefore wasted. When the second washing is not carried out, since residual reducing component interferes with the ionisation of hydrogen the ability to generate power is degraded.
In accordance with the present invention, there is further proposed a process for the production of an active solid polymer electrolyte membrane in a solid polymer type fuel cell comprising a solid polymer electrolyte membrane and a plurality of precious metal catalyst particles supported inside the surface layers of the above-mentioned solid polymer electrolyte membrane by ion exchange and distributed uniformly throughout the inside of the surface layers thereof, wherein the process comprises in sequence a step in which the aforementioned solid polymer electrolyte membrane is immersed in a liquid mixture of a solution of a precious metal complex and at least one additive chosen from the group comprising water-soluble organic solvents, nonionic surfactants and nonmetallic bases so as to carry out ion exchange, a step in which the aforementioned solid polymer electrolyte membrane is washed with pure water, a step in which the aforementioned solid polymer electrolyte membrane is subjected to a reduction treatment, a step in which the aforementioned solid polymer electrolyte membrane is washed with pure water and a step in which the aforementioned solid polymer electrolyte membrane is dried.
As hereinbefore described, when the ion exchange is carried out using a solid polymer electrolyte membrane, which is a polymer ion exchange membrane, under the influence of an additive as mentioned above, precious metal complex ions are adsorbed on a plurality of ion exchange sites which are present inside the surface layers of the aforementioned electrolyte membrane and are distributed uniformly throughout the inside the above-mentioned surface layer. In the first washing step, free precious metal complex ions and the additive which are present inside the aforementioned electrolyte membrane are removed and recovered. In the reduction step, groups of atoms bonded to the precious metal atoms of the precious metal complex ions are removed. In the second washing step the reducing component is removed from the aforementioned electrolyte membrane, and an active solid polymer electrolyte membrane can be obtained via the subsequent drying step.
When the reduction treatment is effected without carrying out the first washing, free precious metal atoms remain inside the aforementioned electrolyte membrane, and as these precious metal atoms generally do not contribute to the generation of hydrogen ions expensive precious metal is therefore wasted. When the second washing is not carried out, since residual reducing component interferes with the ionisation of hydrogen the ability to generate power is degraded.
The above-mentioned objects, other objects, characteristics and advantages of the present invention will be clarified by an explanation of preferable embodiments which are described in detail below by reference to the attached drawings.